With the advent of multimedia data and entertainment services, and the ever increasing popularity of the Internet, the importance of integrated services telecommunication networks ("ISNs") in the future communication infrastructure is fast becoming evident. Current designs for ISNs typically provide for three types of service: constant bit-rate ("CBR"), variable bit-rate ("VBR") and available bit-rate ("ABR"). Present ISN designs provide for CBR service compatible with existing circuit-switched telecommunication networks. Similarly, ABR service is being designed for compatibility with the Internet and Internet-style data transfer applications. However, design considerations with respect to VBR have been dictated by considerations for future telecommunication traffic, with a particular emphasis being placed upon the transmission of compressed video information. This type of video information is generally characterized as having an intrinsic long-term average data rate, punctuated with periods of peak rate data bursts. To facilitate the transmission of such bursty traffic via a standard CBR service network each data burst would have to be smoothed out or reduced via buffering prior to entering the network (causing intolerable delays for real time video signals), or the CBR rate would have to be set at some value that was very close to the peak data rate of the video information being sent (squandering network resources and thereby severely limiting signal multiplexing within the network). Similarly, if such bursty video information is transmitted via an ABR service network, there is no guarantee that the "available" network resources will be sufficient to avoid unacceptable data delays and/or losses. Present designs for VBR network services, such as those discussed by A. E. Eckberg in B-ISDB/ATM Traffic and Congestion Control, IEEE Network, September 1992, pages 28-37, essentially augment standard CBR service with the ability to accommodate moderate data bursts.
To ensure that bursty data transmissions can be carried by a VBR network without unacceptable data delays and/or losses it is essential that the VBR network be provided with an accurate characterization of the data that will be sent. This characterization is communicated to a VBR network via traffic descriptors transmitted along with the data. To maintain data transmission efficiency within a VBR network, it is desirable to provide an accurate characterization of the traffic being sent by using as few traffic descriptors as possible. In practice this has proven to be quite difficult--especially where the data being sent is compressed video.
Compressed video data simply does not conform to the "moderately bursty" traffic model envisaged by designers of VBR service networks. As is well known in the art, compressed video data typically includes fairly long intervals (on the order of tens of seconds) where the data rate is very near what would have been considered the peak rate for the typical VBR model (see E. P. Rathgeb, Policing of Realistic VBR Video Traffic in an ATM Network, International Journal of Digital and Analog Communications Systems, vol. 6, pages 213-26, 1993; M. W. Garrett and W. Willinger, Analysis Modeling and Generation of Self-Similar VBR Video Traffic, ACM Sigcomm '94, pages 269-80, University College London, August 1994). These extended high-rate data bursts are due to scenes depicting considerable motion and/or quickly varying light levels. For such traffic, if a leaky-bucket type of traffic descriptor is used, one is faced with a series of poor choices.
For example, assume that the video data traffic is being routed through the system illustrated in FIG. 1. As shown, video data is sent from network subscriber site 100 to remote user location 101 via VBR network 102. In response to signal from processor 103, data is transmitted from compressed video source 104 to VBR network 102 by way of source buffer 105 and regulator 106. Regulator 106 is a "leaky-bucket" data regulator, a type that is well-known in the art. This type of regulator allows data to be output at a particular rate as a function of the availability of data tokens (107) within token bucket register 108. Tokens are "placed" in token bucket register 108 at a predetermined rate, and depleted as data passes through regulator 106--When token bucket 108 is empty, no additional data is permitted to pass through regulator 106. These tokens are virtual in nature; that is, they only serve to meter data flow through regulator 106, and are not inserted into the outgoing data stream. If the token availability/data rate of regulator 106 is chosen so that the rate of data output from regulator 106 approximates the average data rate at which data leaves compressed video source 104 (a condition that will maximize the statistical multiplexing gain within VBR network 102), and if the size of token bucket register 108 (i.e., the maximum number of tokens that may be held in this register) is fixed at a moderate level (so as to avoid overloading VBR network 102), then source buffer 105 will have to be very large in order to support an extended high-rate data burst from compressed video source 104. Barring the availability of such a large source buffer, data losses will occur. Even if source buffer 105 is made large enough to handle such sustained bursts of peak video transmission, the result is still far from ideal--Data losses will be avoided, but, due to the large source buffer, equipment expenses increase and long delays will be experienced with respect to source output.
Alternatively, if token bucket register 108 is made large enough to allow token regulator 106 to rapidly drain source buffer 105 of data gluts resulting from sustained video data bursts, then large network and remote user location buffers (109, 110) will be needed to avoid data losses and ensure proper delivery of a usable video signal to receiver/viewer 111. Furthermore, by allowing such bursts to be freely drained and launched into VBR network 102 a single network subscriber site is given the ability to disrupt VBR network 102 by flooding it with tens of megabytes of data.
Thus, the phenomenon of sustained peaks of high-rate data will result in either high data losses, large delays between source and recipient, or a disruption-prone unregulated VBR network environment. Given the current framework of VBR network service, there is no clear way to avoid all of these problems simultaneously. This is a simple consequence of the fact that the sustained peaks exhibited in compressed video data violate the basic design assumptions for VBR service.